1. Field of the Invention
The present invention is related to a printed wiring board having a through-hole that penetrates an insulative substrate, and to a method for manufacturing such a wiring board; especially, to a printed wiring board in which the through-hole is filled with metal, and to a method for manufacturing such a wiring board.
2. Discussion of the Background
Japanese Laid-Open Patent Publication 2004-311919 describes technology (a method for filling through-holes) to avoid occurrence of voids or the like by configuring a through-hole having a cross-sectional shape resembling that of an hourglass. Such a through-hole configuration indicates a through-hole made up of a first opening tapering from the first surface of the substrate toward the second surface of the substrate and of a second opening tapering from the second surface of the substrate toward the first surface of the substrate. The first opening and the second opening are connected inside the substrate.
In such a through-hole filling method, as shown in FIG. 8A, through-hole 801 is first formed in insulative substrate 800. Next, as shown in FIG. 8B, seed layer 802 made of metal is formed on the inner-wall surface of through-hole 801. Then, electrolytic plating is performed to form plated-metal layer 803 by filling metal starting in the narrowest portion (the portion whose lateral cross-section is the smallest).
It is thought that when a reinforcing material protrudes from the wall surface of the resin as shown in FIG. 9A, the hole diameter (L1) of the opening which penetrates reinforcing materials (810a, 810b) could become smaller than the diameter (L2) of the narrowest portion of through-hole 801 that penetrates the resin. When a substrate is made of two layers of reinforcing material and resin, as shown in FIG. 9A, and when one reinforcing material (810a) is positioned between the first surface of the substrate and the middle section of the substrate and the other reinforcing material (810b) is positioned between the second surface (the surface opposite the first surface) and the middle section of the substrate, it is thought that areas penetrating reinforcing materials (810a, 810b) may be blocked by plating (electrolytic plated film) before the narrowest portion of through-hole 801 penetrating the substrate is filled. As shown in FIG. 9B, it is thought that there is a risk that void 811 occurs in the narrowest portion of through-hole 801 that penetrates the resin. The contents of this publication are incorporated herein by reference in their entirety.